The main circuits of electrical devices such as household electrical appliances, audio devices, and computers are printed circuit boards to which electronic parts are soldered. An age ago, a Sn—Pb solder having a composition of Sn-37 mass % Pb (below, % indicates mass %) was used for soldering of printed circuit boards. However, discarded electrical devices were often buried underground, and Pb dissolved from the discarded printed circuit board and sometimes polluted underground water.
In light of this environmental problem, the use of lead-free solders for printed circuit boards has been increasing for more than 10 years, and today, almost all electrical devices are made using lead-free solder.
Conventionally used Sn—Pb solder has a melting point of 183° C., by which it has the advantage that soldering can be performed at a low operating temperature. If the operating temperature for soldering is low, it is not necessary to be concerned about the heat resistance of electronic parts used in soldering, and it is possible to use electronic parts having low heat resistance such as plastic connectors.
In contrast, the principal lead-free solders currently used are Sn—Ag—Cu based lead-free solders such as Sn-3.0% Ag-0.5% Cu. Sn—Ag—Cu based lead-free solders have the good property that they have greater heat cycle resistance than conventional Sn—Pb solder. However, as their melting temperature is around 220° C. which is roughly 40° C. higher than that of a conventional Sn—Pb solder, electronic parts having low heat resistance could not be used.
When lead-free solders were first developed, because there were many electronic parts which did not have heat resistance, the solder alloys which were developed for use were low-temperature solders such as Sn-58% Bi solder (melting temperature of 139° C.), which had been conventionally used as low-temperature solder, and a solder alloy having increased strength and elongation by adding a small amount of Ag to Sn-58% Bi solder (JP H08-252688 A=Patent Document 1).
However, it was observed that if these Sn—Bi based solder alloys are used with small electronic devices such as mobile phones or notebook computers, when the small electronic devices are dropped, they have low resistance to drop impacts and fracture at the solder bonding interface. Therefore, these solder alloys were not used for such electronic devices.
In order to obtain a solder connection structure having resistance to fatigue, a method which alleviates shear stresses by applying a Sn—Pb solder paste to the bonding surface of a printed circuit board, disposing an In-containing solder ball in contact with the solder paste, and heating the paste and the preform has been disclosed (JP H08-116169 A=Patent Document 2).
Patent Document 1: JP H08-252688 A
Patent Document 2: JP H08-116169 A